1. Field of the Invention
This invention relates to a hydraulic control device for use with an automatic speed change gear used in a diesel engine on a vehicle, which engine is provided with a torque converter and an auxiliary speed change gear device, and more particuarly to the control of a back-pressure for an accumulator for use with an automatic speed change gear device used in a diesel engine.
2. Description of the Prior Art
Hitherto, an automatic speed change gear device used with diesel engine suffers from disadvantage of an excessively great shock when changing a speed, as compared with a gasoline combined for use with an automatic speed change gear device. This is often attributable to the fact that a hydraulic control system for use with the automatic speed change device in a gasoline engine is applied to the diesel engine in fact, despite the different characteristics between the diesel engine and the gasoline engine.
FIG. 3 shows the typical relationship among the R.P.M., output torque and throttle openings, of an gasoline engine. In contrast to this, FIG. 4 shows similar characteristic curves of a diesel engine according to a pnemuatic governor control system. (This will be referred to simply as a diesel engine, hereinafter). As is best shown in FIGS. 3 and 4, an almost constant maximum torque may be obtained irrespective of the extents of stepping on an accelerator pedal in the case of a diesel engine, whereas the maximum torque to be obtained will be greater with an increase in a throttle opening in the case of a gasoline engine. On the other hand, with the diesel engine, the greater the extent of stepping on an accelerator pedal, the higher will be the maximum R.P.M. However, with a gasoline engine, a considerably high R.P.M. results at a smaller extent of stepping on the accelerator pedal, and thus the maximum R.P.M. is not dependent on throttle openings. Furthermore, with a diesel engine, a substantially constaant torque may be obtained up to a certain R.P.M. at a given step angle of the accelerator pedal, while the torque will sharply drop when a certain R.P.M. is exceeded. In contrast thereto, with a gasoline engine, the torque will drop smoothly.
Meanwhile, there has been recently used an accumulator for reducing or moderating a shock due to an automatic speed change gear device, wherein a line pressure is directly used as a back pressure for the accumulator. As shown in FIG. 5, the line pressure will gradually increase with the increase in throttle opening. FIG. 6 illustrates the relationship between an engaging pressure required for engaging a friction member of an automatic speed change gear device in a gasoline engine, wherein the larger the throttle opening, the higher will be the engaging pressure, accordingly. As a result, if only the line pressure is directly used as a back pressure for an accumulator, a resulting shock may be effectively remedied. Nevertheless, in the case of an automatic speed change gear device for use in a diesel engine, there should be used an engaging hydraulic pressure as shown by the solid line in FIG. 7 due to the characteristics inherent to the diesel engine. As is best shown in FIG. 7, the engaging pressure required is maintained lower than the line pressure P.sub.L in the range of zero to .theta..sub.o, of the throttle openings. If, however, the line pressure as shown by the broken line but within the above range is fed as a back pressure for an accumulator, then there will result a considerably great shock due to the change in speed. On the other hand, the use of an excessively high engaging pressure exerts adverse effects on a friction engaging member.